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Date: Tue, 2 Nov 2021 22:48:19 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Ard Biesheuvel <ardb@...nel.org> Cc: Sami Tolvanen <samitolvanen@...gle.com>, Mark Rutland <mark.rutland@....com>, X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Sedat Dilek <sedat.dilek@...il.com>, Steven Rostedt <rostedt@...dmis.org>, linux-hardening@...r.kernel.org, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev, joao@...rdrivepizza.com Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching On Tue, Nov 02, 2021 at 07:18:53PM +0100, Ard Biesheuvel wrote: > > The range check isn't fundamental to CFI, having a check is the > > important thing AFAIU. > > Agreed. If the call site has a direct branch, it doesn't need the range check. That, from the earlier email: | And have the actual indirect callsite look like: | | # r11 - &foo | ALTERNATIVE_2 "cs call __x86_indirect_thunk_r11", | "cs call __x86_indirect_cfi_deadbeef", X86_FEATURE_CFI | "cs call __x86_indirect_ibt_deadbeef", X86_FEATURE_IBT So the callsite has a direct call to the hash-specific and cfi-type specific thunk, which then does an (indirect) tail-call. The CFI one does the hash check in the thunk and jumps to the function proper, the IBT one on does it in the landing-pad. The !CFI one ignore it all and simply does an indirect call (retpoline aided or otherwise) to the function proper -- in which case we can free all the thunks.
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